In this test, the experimental materials are root tips of Solanum nigrum var. suaveolens and Solanum nigrum. In order to determine pretreatment methods of the most suitable for karyotype analysis, we optimized processing the root tips by different pretreatment, fixed and different dissociation. Root tips of Solanum nigrum L. are pretreated with 0.05% colchicine for 2.5 h at 15℃. Then root tips were hydrolyzed or 6 min in 1 mol/L HCl at 60 ℃. The results showed the chromosome clearer. S. nigrum var. suaveolens and S. nigrum are both belong to small chromosomes, S. nigrum var. suaveolens is tetraploid, the karyotype formula is K (4n) =48=4sm+44m, karyotype asymmetry belongs to 2B. The karyotype asymmetry coefficient was 56.22%. S. nigrum L. is hexaploid, the karyotype formula is K (6n) =72=72m, karyotype asymmetry belongs to 1B and the karyotype asymmetry index was 55.89%.